Laser diode package with enhanced cooling

ABSTRACT

A laser diode package assembly includes a reservoir filled with a fusible metal in close proximity to a laser diode. The fusible metal absorbs heat from the laser diode and undergoes a phase change from solid to liquid during the operation of the laser. The metal absorbs heat during the phase transition. Once the laser diode is turned off, the liquid metal cools off and resolidifies. The reservoir is designed such that that the liquid metal does not leave the reservoir even when in liquid state. The laser diode assembly further includes a lid with one or more fin structures that extend into the reservoir and are in contact with the metal in the reservoir.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.12/693,052, filed Jan. 25, 2010, the disclosure of which is incorporatedby reference herein in its entirety for all purposes.

This application is related to U.S. patent application Ser. No.13/198,393 filed on Aug. 4, 2011 and U.S. patent application Ser. No.13/198,414 filed on Aug. 4, 2011.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH OR DEVELOPMENT

The United States Government has rights in this invention pursuant toContract No. DE-AC52-07NA27344 between the U.S. Department of Energy andLawrence Livermore National Security, LLC, for the operation of LawrenceLivermore National Laboratory.

BACKGROUND OF THE INVENTION

Laser diode arrays are used in a wide range of commercial, medical andmilitary applications: materials processing (soldering, cutting, metalhardening), display technology/graphics, medical surgical procedures(corneal shaping, tissue fusion, dermatology, photodynamic therapy),satellite communication, remote sensing and laser isotope separation. Incertain solid-state laser applications it is desirable to use laserdiode arrays to optically excite, i.e., “pump,” the crystal hosts.Diodes offer a narrow band of emission (reducing thermal lensing),compactness, high electrical efficiency and higher reliability ascompared to flash lamps. Despite these numerous advantages, however,diode-pumped solid-state lasers (DPSSLs) have gained slow marketacceptance due to the high cost associated with the laser diode arraypumps. Significant diode array cost reductions would enable widedeployment of DPSSLs and new architectures to be realized that werepreviously cost prohibitive. In particular, low-cost diode arrays wouldbolster the inertial confinement fusion (ICF) and inertial fusion energy(IFE) programs that require low-repetition rate laser diode arrays invery high volumes.

Laser diode arrays dissipate a large amount of heat that needs to beeffectively channeled away from the diodes. One of the methods tocontrol the costs of the laser diodes is to generate more optical powerout of each diode. As the amount of optical power outputted from eachdiode increases, the cost of running a laser diode array decreases—i.e.the cost per watt of power decreases. However, the downside to runningthe laser diodes at increased optical output is that they generateenormous amounts of heat that need to be dissipated. Consequently,advanced heat abatement mechanisms are needed in order to run laserdiode arrays at increased outputs. This in-turn increases the cost ofpackaging a laser diode since more effective cooling of the laser diodesis needed.

What is needed is a low cost package that effectively dissipates theheat generated by the laser diode when the laser diode is operated atelevated optical output power.

SUMMARY OF THE INVENTION

This disclosure generally relates to device packaging. Morespecifically, the disclosure relates to techniques for providingadvanced heat dissipation in a laser diode package.

Certain embodiments of the present invention provide a laser diodepackage for improving the heat dissipation from the laser diode. Thepackage includes a substrate having a lower surface, an upper surface,and a height. The package further includes a region disposed within thesubstrate. The region includes a fusible material that has a solid stateand a liquid state. The region has an upper surface and an opposinglower surface, wherein the upper surface of the region is flush with theupper surface of the substrate and the lower surface of the region isseparated from the upper surface of the region by a first distance thatis less than the height of the substrate. A cover plate is disposed onthe substrate such that the bottom surface of the cover plate is incontact with the upper surface of the substrate and the upper surface ofthe region. In addition, the package includes a laser diode coupled tothe top surface of the cover plate. During the operation of the laserdiode, the fusible material melts and transitions into the liquid state.In the liquid state the fusible material is confined within the region.In some embodiments, the fusible material can include non-metallicmaterials like paraffin.

Other embodiments of the present invention provide an array of laserdiode chips. The laser diode chip array includes a substrate having anupper surface and a lower surface and a plurality of v-grooves areformed in the upper surface. A plurality of laser diode bar assembliesis placed within the v-grooves, wherein a single laser diode bar isplaced in each of the v-grooves. Each laser diode bar assembly furtherincludes a thermal plate having a height and a width, a cavity disposedwithin an upper region of the thermal plate, the cavity having a firstpredetermined height and a second predetermined width and at leastpartially filled with fusible metal. The second predetermined width ofthe cavity is less than the width of the thermal plate and the firstpredetermined height of the cavity is less than the height of thethermal plate. The laser diode assembly further includes a cover platedisposed on the thermal plate and a laser diode bar coupled to the coverplate.

In yet other embodiments, the laser diode package includes a verticallyoriented mount substrate having a first side surface and a second sidesurface, a reservoir filled with a fusible metal disposed within themount substrate and located near the first side surface of the mountsubstrate. The reservoir is characterized by a volume. A laser diode baris attached to the first side surface of the mount substrate and a metalplate is attached to the laser diode bar.

In some embodiments, a laser diode package is provided. The laser diodepackage includes a mount substrate having a lower surface, an uppersurface, and a first height. The laser diode package further includes aregion disposed within the mount substrate and along the upper surfaceand the region is filled with a fusible metal wherein the region has asecond height that is less than the first height of the substrate. Acover plate having a top surface and a bottom surface is disposed on thesubstrate. The cover plate includes a plurality of fin structures alongthe bottom surface wherein the plurality of fin structures extend intothe region and are in contact with the fusible metal. A laser diode baris coupled to the top surface of the cover plate.

The following detailed description, together with the accompanyingdrawings will provide a better understanding of the nature andadvantages of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates laser diodes placed in a series of v-grooves of aconventional micro-channel cooled sub-mount;

FIG. 2 illustrates laser diodes with a modified heat dissipationmechanism according to a first embodiment of the present invention;

FIG. 3 is a simplified cross-sectional view of a laser diode assemblyaccording to a second embodiment of the present invention;

FIG. 4 is a simplified cross-sectional view of a laser diode assemblyaccording to a third embodiment of the present invention;

FIG. 5 shows a table listing the thermal properties of Gallium used invarious embodiments of the present invention;

FIG. 6 is a graph illustrating spatial temperature profile of a laserdiode package according to an embodiment of the present invention;

FIG. 7 is a table illustrating the correlation of lid thickness toreduction in junction temperature of a laser diode package according toan embodiment of the present invention;

FIG. 8 is a table illustrating the correlation of lid thickness andreservoir height to reduction in junction temperature of a laser diodepackage according to an embodiment of the present invention;

FIG. 9 is a flow diagram for a process for operating a laser diodeaccording to an embodiment of the present invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Embodiments of the invention generally relate to laser diodes andspecifically to methods and a system for cooling a laser diode assembly.

FIG. 1 shows a plurality of laser diode bars placed onto a microchannelcooled type sub-mount in a series of v-grooves according to aconventional design. FIG. 1 shows the electrical circuitry, andmicrolens placement. Water manifold 10, comprising inlet ports 12 andexit ports 14, is connected to the angular groove microchannel cooler16. In the figure, water enters and exits angular groove microchannelcooler 16 through inlet ports 12 and exit ports 14, respectively.Angular groove microchannel cooler 16 comprises metalization layer 18,which has its electrical continuity broken by electrical isolation break20. Laser diode bars 22 are located against the metalization layer 18and are soldered into place. The electrical conduction path is completedwith Copper foil fingers 24 coupling the top side of the laser diodechip to the metallization layer 18. Microlenses 26 are located inproximity to the output face of laser diode bars 22 such that the outputbeam 28 is collimated.

Most of the conventional cooling techniques involving use of a liquid tocool devices such as laser diodes involve circulation of the liquid in aclosed loop system, e.g., as illustrated in FIG. 1 above. Examples ofsuch liquid cooling mechanisms include micro-fluidic channels thatcirculate a cooling liquid, and the like. In sum, most of the coolingtechniques in use today are focused on delivering the cooling liquidnear the surface to be cooled, transferring the heat to the liquid,transporting the heated liquid away from the surface, cooling theliquid, and delivering the cooled liquid back near the surface.Implementing such a cooling scheme requires added infrastructure ofcreating a closed loop system for the fluid to circulate. While this maynot be a problem when there is only one device or few devices that needthe cooling, however, it may result in significant cost impact whenhundreds of thousands of devices need cooling at the same time. Forexample, a laser diode array used in a nuclear fusion engine may includemillions of laser diodes. Using a closed loop liquid cooling system inclose proximity to the laser diodes for such an array may not bepractical or feasible.

Embodiments of the present invention relate to methods for cooling alaser diode assembly. In some embodiments, the laser diode has a shortduty cycle. For example, the duty cycle of the laser diode can rangebetween 1% and 10%. Duty cycle of a laser diode is the proportion oftime during which the laser diode is operated or turned ‘on’. Forexample, if a laser diode operates for 1 second, and is shut off for 99seconds, then it is operated for 1 second again, and so on, then thelaser diode is operational for one out of 100 seconds, or 1/100 of thetime, and its duty cycle is therefore 1/100, or 1 percent. There is aneed to effectively channel the heat away from the laser diode when thelaser diode is “on.” In some embodiments, a fusible metal is placed inclose proximity to the laser diode. When the diode is turned on, theheat generated by the laser diode will result in a phase change of thefusible metal, e.g., solid to liquid. During this phase change, thefusible metal absorbs a large amount of heat thus effectively channelingthe generated heat away from the laser diode. During the period when thelaser diode is off, the fusible metal can transfer the absorbed heat toother heat abatement systems, e.g. a heat sink, and resolidify again. Insuch an instance, the added expense and complexity of circulating liquidcooling systems can be avoided thus reducing the packaging costs for thelaser diode.

FIG. 2 illustrates a cross-sectional view of a laser diode assembly 200according to an embodiment of the present invention. Assembly 200includes a heat spreader structure 210. In some embodiments, heatspreader structure 210 is in the form of a metal plate. In someembodiments, the heat spreader structure 210 is made from materialsincluding Copper and Tungsten. Other suitable materials may also be usedbased on the application, e.g., ceramic, Beryllium oxide, or Aluminumoxide. In some embodiments, heat spreader 210 has a thickness (height)of about between 250 μm and 2000 μm and a width of about between 500 μmand 1 cm. Heat spreader 210 has a cavity 240 formed along an uppersurface 211 of the heat spreader. Cavity 240 has a thickness (height)that is less than the thickness of heat spreader 210 and a width that isless than the width of the heat spreader. Thus, the cavity is containedwithin the heat spreader. In some embodiments, the height of cavity 240is between 40 μm and 200 μm. In an exemplary embodiment, the height ofcavity 240 is about 50 μm. Cavity 240 is filled with a fusible metal 250that can undergo phase change with absorption of heat. In someembodiments, cavity 240 is filled with a fusible metal, e.g., Gallium. Alid 230 is disposed over cavity 240 and surface 211 of heat spreader 210such that lid 230 seals cavity 240. Lid 230 is manufactured frommaterials including but not limited to Copper, Diamond, Tungsten,Aluminum Nitride, and combinations thereof In some embodiments, theheight or thickness of lid 230 is between 300 μm to less than 100 μm. Alaser diode 220 is mounted on top of lid 230. Laser diode 220 can be alaser diode comprising epitaxial layers ofindium-Gallium-Aluminum-Arsenide-phosphide (InGaAlAsP) grown on aGallium-Arsenide (GaAs) substrate.

In operation, the laser diode is turned on and off according to the dutycycle required by the application. As discussed above, the laser diodeis operated in a short duty cycle mode. For example, in one embodiment,the laser diode is operated in a pulsed mode with the laser diode beingoperated with 350 μs bursts. When the laser diode is turned on, thelaser diode generates a significant amount of heat. Most of the heat isgenerated extremely close to the interface between laser diode 220 andlid 230. In one embodiment, the laser diode generates up to 5 kW/cm² ofheat during a single burst. In one embodiment, the temperature rise oflaser diode 220 is minimized when laser diode 220 is operational. Inthis embodiment, during operation of laser diode 220, the heat generatedby the laser diode is absorbed by the Gallium and causes the Gallium incavity 240 to melt. This helps to maintain the temperature of laserdiode 220 at a constant level. When the laser diode is off, the Galliumre-solidifies after transferring the absorbed heat to heat spreader 210and surrounding structures. However, the Gallium stays within the cavityeven when it is melted and does not leave cavity 240.

In one embodiment, Gallium undergoes two phase transformations duringeach cycle of the laser diode operation. First, when the laser diode ison, the Gallium undergoes a first phase transformation from a solidphase to a liquid (molten) phase. Second, during the idle mode or whenthe laser diode is off, the Gallium metal undergoes a second phasetransformation from the liquid (molten) state back to a solid state. Insome embodiments, the Gallium may never re-solidify completely but mayonly attain a partially solid state. FIG. 5 shows a table 500illustrating properties of Gallium during various phases of the Gallium.As shown in table 500, thermal properties of a material depend on thedensity (ρ (rho)), specific heat capacity (C), and thermal conductivity(K) of the material. More specifically, metals with a high value for theproduct of the density, specific heat capacity, and thermal conductivityof a material (ρCK) have better heat conduction capability in short dutycycle applications. As illustrated in table 500, Gallium has relativelylow specific heat capacity during the solid and the liquid phases;however, during the phase transition from solid to liquid, Galliumbehaves like a material with a substantially high specific heat capacitydue to its latent heat of fusion, thus increasing its heat absorptioncapacity significantly. Gallium is one of the metals that can be usedeffectively to conduct heat away from the laser diode. One skilled inthe art will realize that there are other metals that may also be used,e.g., alloys of Gallium, Bismuth, Indium, and Tin. In some embodiments,the temperature at which the high heat capacity is achieved can beadjusted over a significant temperature range by varying the compositionof the fusible metal. In some embodiments, the composition of thefusible metal determines its melting temperature. For example, themelting temperature of the fusible metal can be adjusted between 14° C.(287 K) to above 40° C. (313 K) by varying the composition of thefusible metal. The requirement for having a particular meltingtemperature for the fusible metal may depend on the application and dutycycle of the laser diode.

It is to be noted that cavity 240 need not be filled only with a fusiblemetal. In some embodiments, a combination of fusible and non-fusiblemetal may be used. For instance, in one embodiment, instead of justGallium, a composite structure including Copper and Gallium can be used.In this instance, a Copper plate with pores is created. The structure ofthe Copper plate is similar to foam. Gallium can then be embedded in thepores of the Copper plate. This creates a composite structure thatcomprises Copper with Gallium (or other suitable material) embeddedtherein. One of the advantages of this type of structure is that sinceCopper is a good conductor of heat, it helps to carry the generated heatfrom the laser diode to solid metal in regions farther from the laserdiode in order to aid the Gallium in absorbing the heat. So in effect,the Copper acts as a heat conduit to effectively and uniformly channelthe heat to the Gallium so that Gallium may perform its functioneffectively.

In some embodiments, cavity 240 can be filled with a solid paraffinfusible material. For materials that have increased heat absorptioncapacity during their phase transition from solid to liquid, it isadvantageous to always have the solid material in contact with or inclose proximity to the surface generating the heat. In this instance, itwould be beneficial to have a portion of the solid fusible material inclose proximity to the laser diode so that the portion of the solidfusible material closest to the laser diode junction will melt thusabsorbing the heat generated by the laser diode. In one embodiment, thefusible material within cavity 240 can be circulated so as to have somesolid portion of the fusible material in close proximity of the laserdiode junction at any given time. This will increase the heat absorptioncapacity of the fusible material. In order to accomplish this, in anembodiment, when the solid fusible material near the laser diodejunction melts it is carried away from the laser diode junction, e.g.,towards the bottom of cavity 240. This forces the solid material thatwas at the bottom of cavity 240 to move up and come in close proximityof the laser diode junction. While the solid material, which is pushedup, is melting due to absorption of heat the liquid material that ispushed down towards the bottom of cavity 240 can cool, resolidify, andbe ready to be pushed up towards the laser diode junction. This createsa closed loop system within cavity 240 such that there is always somesolid portion of the fusible material in close proximity to the laserdiode junction. This increases the efficiency of the fusible material incarrying heat away from the laser diode by taking advantage of theconstant phase change occurring at the laser diode junction.

FIG. 3 illustrates a laser diode package assembly 300 according toanother embodiment of the present invention. In this embodiment,assembly 300 includes a substrate 302. Substrate 302 is verticallyorientated and may include Copper or an alloy including Copper.Substrate 302 acts a heat sink to conduct the heat away from the laserdiode. A lid 306 is coupled to substrate 302. An upper portion of lid306 includes a reservoir 312. Reservoir 312 is embedded in lid 306 andcan be formed using any of the conventional techniques. Reservoir 312 isat least partially filled with a fusible metal 304. Metal 304 is in asolid state or partially solid state when the laser diode is in the idlestate. Metal 304 absorbs the heat generated by laser diode 220 andchanges its phase from solid to liquid. However, since the reservoir isvertically oriented, the melted metal 304 does not leave the reservoirdue to gravity. Reservoir 312 is not completely filled with metal 304 inorder to accommodate the expansion of metal 304 during its phasetransition from solid to liquid. In some embodiments, metal 304 isGallium. In other embodiments, metal 304 may comprise a compositestructure of a fusible and non-fusible metal, e.g., Gallium and Copper,or Copper foam with Gallium embedded in the Copper foam. The Copper foammay be formed using multiple interwoven Copper filaments. In someembodiments, lid 306 is made from an alloy including Copper and Tungstenor an alloy including Copper and Diamond. A laser diode 220 is coupledto lid 306. The heat generated by laser diode 220 is coupled tosubstrate 302 via lid 306. In some embodiments a contact plate 308 isconnected to laser diode 220. In some embodiments, contact plate 308includes Copper. In operation, contact plate 308 provides the electricalpower needed for operation of laser diode 220.

In operation, when laser diode 220 is on, the heat generated by laserdiode 220 is absorbed by metal 304 which results in metal 304 changingits state from a solid phase to a liquid phase. However, since reservoir312 is vertically oriented, the melted metal 304 does not leave thereservoir and the reservoir has enough headspace to accommodate anyexpansion of the metal as it undergoes the phase change. When laserdiode 220 is off, metal 304 cools and resolidifies by channeling theheat to substrate 302. In the instance where metal 304 is Gallium, it isknown that if Gallium is cooled slowly below its melting point, it doesnot re-solidify unless it is seeded with solid material. In someembodiments, reservoir is designed in such as manner that the Gallium inthe reservoir does not completely melt. Thus, some amount of solidGallium is always available to initiate the resolidifying process whenthe laser diode is off. In some embodiments, the resolidificationprocess may occur in between 1 μs to 3 μs. In another embodiment, inorder to enable the rapid exchange of heat from the laser diode to themetal, metal 304 may be confined in the reservoir using a flexiblemembrane at the interface between the laser diode and the lid. In someembodiments, the membrane can be made from material including Siliconerubber.

In some embodiments, the top of reservoir 312 is covered with a platehaving an opening to provide further confinement of the Gallium and toprevent contamination caused by foreign particles entering thereservoir. In one embodiment, the plate can be sealed once the fusiblemetal is introduced into the reservoir via the opening in the plate.

FIG. 4 illustrates a laser diode package assembly 400 according to yetanother embodiment of the present invention. Assembly 400 includes aheat sink 402. Heat sink 402 can be made from Copper or any othersuitable material. Heat sink 402 serves to conduct heat away frommaterials to which heat sink 402 is coupled. A reservoir 404 is formedin an upper portion of heat sink 402. Reservoir 404 is filled with ametal that can undergo a phase change from solid to liquid uponabsorption of heat. In some embodiments, reservoir 404 is filled withGallium, although other suitable metals may also be used as describedabove. A lid 406 is coupled to heat sink 402 such that it is disposedover reservoir 404 and heat sink 402. In some embodiments, reservoir 404is formed such that it is completely contained between heat sink 402 andlid 406. Lid 406 includes a plurality of “fin” structures 416 protrudingfrom a bottom surface of lid 406 and partially extending into reservoir404. Fin structures 416 extend to a fixed distance into reservoir 404.In some embodiments, fin structures 416 are between 30 μm and 50 μmthick and extend to distance of between 100 μm and 250 μm into reservoir404. In an exemplary embodiment, fin structures 416 are 40 μm thick andextend to a distance of about 200 μm into the reservoir. The distance towhich the fin structures extend into the reservoir is calculated fromthe top surface of the reservoir. In some embodiments, fin structures416 are spaced apart by a distance of between 10 μm to about 70 μm. Insome embodiments, the center-to-center distance between two adjacentfins is between 50 μm and 100 μm. In some embodiments, the number of finstructures 416 can range between 50 and 250. Fin structures 416 reducethe transport distance for the heat and increase the effective surfacearea for heat transport into reservoir 404.

A laser diode 220 is attached to lid 406 using a first solder material414. In some embodiments, first solder material 414 includes Gold andTin. In some embodiments, a metal plate electrode 410 is attached tolaser diode 220 using a second solder material 412. In some embodiments,second solder material 412 includes Indium. Metal plate 410 is used toprovide electrical connection to laser diode 220. In some embodiments,heat sink 402 is coupled to lid 406 using an Indium-based soldermaterial.

The reservoir including the heat absorbing metal is kept as close to thelaser diode junction as possible. As the distance between the reservoirand the diode junction increases, the reservoir becomes less effectiveat removing heat due to the increasing thermal impedance from thereservoir to the junction. It is beneficial if the reservoir can beplaced in close proximity to the diode junction. One way ofaccomplishing this is to make the lid as thin as possible while stillmaintaining the structural integrity required for mounting the laserdiode. In some embodiments, the lid thickness is maintained below 100μm. In another embodiment, the lid can be fabricated using high thermalconductivity materials, e.g., Copper, Diamond, Graphite, etc. in orderto minimize the thermal impedance of the lid. In some embodiments, thereservoir can be plated with metals including nickel or coated withDiamond-like carbon material to prevent the corrosion of the surroundingmaterials due to Gallium. In some embodiments, the reservoir may beformed using materials such as, silicon, beryllium oxide, or varioustypes of ceramics.

The choice of heat absorption metal, the relative heights of thereservoir and the lid, and the operation mode of the laser diodesignificantly impact the manner in which the laser diode can be cooled.It is not merely a matter of design choice to arrive upon the dimensionsand choice of materials. It would not be obvious to one skilled in theart to arrive upon the described dimensions and materials withoutextensive study of the materials and exhaustive analysis of the impactof these materials and dimensions on the cooling efficiency for thelaser diode. For example, FIG. 6 illustrates a graph that shows thespatial temperature profile of a laser diode assembly after a 360 μs,500 W heat pulse with a 100 μm lid and when using no Gallium and using a200 μm thick Gallium reservoir. The vertical lines in the graph showinterfaces between different materials, e.g., Cu, CuW, GaAs, Ga, Cu,etc. As seen in FIG. 6 a reduction in junction temperature, ofapproximately 3° K is attained when using a 200 μm thick Galliumreservoir. By further adjusting the thicknesses of the Gallium reservoirand the lid, a reduction in junction temperatures of about 7° K can berealized.

As described above, as the solid/liquid interface of the Gallium issituated farther from the diode junction, the reservoir becomes lesseffective at removing heat due to the increasing thermal impedance fromthe solid Gallium in the reservoir to the junction. The effect of themoving solid boundary causes a complex time dependence of the junctiontemperature, which depends on the magnitude of the heat load. At shorttimes, before the reservoir begins to melt, a laser diode assemblywithout a reservoir performs better due to the better thermal behaviorof, e.g., the CuW lid, as compared to Gallium. But, as the Galliumbegins to melt, a sharp reduction in the rate of junction temperaturerise occurs, and shortly thereafter a laser diode assembly with areservoir shows improved performance, e.g., reduced junctiontemperature. In some embodiments, the bottom heat sink, e.g., heat sink402 of FIG. 4, is operated at a temperature below “ambient” to increasethe thermal gradient and heat flow across the Gallium.

FIG. 7 shows a table 700 illustrating the correlation of the lidthickness to reduction in junction temperature according to anembodiment of the present invention. In table 700, the thickness of theGallium reservoir is kept constant, e.g., 200 μm, while the thickness ofthe lid is varied. As seen from table 700, thinner lids provide the bestthermal performance. In general a lid with thickness less than 100 μm isdesirable. FIG. 8 shows a table 800 illustrating junction temperaturereduction for a 250 W heat pulse and various heights of reservoirs andthicknesses of the lids. As seen in table 800, a thin reservoir and athin lid provide enhanced heat dissipation capability and result inimproved performance. However, practical aspects like manufacturabilityof the lids and the reservoirs may limit the thermal performance thatcan be obtained. In some embodiments, a reservoir depth of less than 250μm and a lid thickness of less than 100 μm may be used.

FIG. 9 illustrates a flow diagram for a process 900 of operating a laserdiode according to an embodiment of the present invention. At step 901,a reservoir is provided in close proximity to a laser diode. Thereservoir is at least partially filled with a solid fusible material,e.g., Gallium. At step 902, the laser diode is operated in a pulsedmode, e.g., with a low duty cycle. At step 903, the fusible material inthe reservoir melts as it absorbs the heat generated by the laser diode.The material undergoes a phase transition from solid to liquid. However,the liquid material does not leave the reservoir. As a result of theheat absorption by the fusible material in the reservoir, heat isconducted away from the laser diode at step 904 and the laser diode iscooled. At step 905, the laser diode is turned off. The cooling of thelaser diode continues after the laser diode is turned off. At step 906,the fusible material in the reservoir resolidifies, e.g., by conductingthe absorbed heat to a heat sink as a result of the cooling. Asexplained earlier, not all the solid material in the reservoir may melt.Thus, the remaining solid material provides a seed for the remainingmelted material to resolidify. After the fusible material resolidifies,it is again ready to absorb heat generated by the laser diode during thelaser diode's next operating cycle.

It should be appreciated that the specific steps illustrated in FIG. 9provide a particular method of operating a laser diode according to anembodiment of the present invention. Other sequences of steps may alsobe performed according to alternative embodiments. For example,alternative embodiments of the present invention may perform the stepsoutlined above in a different order. Moreover, the individual stepsillustrated in FIG. 9 may include multiple sub-steps that may beperformed in various sequences as appropriate to the individual step.Furthermore, additional steps may be added or removed depending on theparticular applications. One of ordinary skill in the art wouldrecognize many variations, modifications, and alternatives.

Various advantages are realized by the embodiments of the presentinvention. For instance, a latent heat reservoir, as described above,can reduce the maximum temperature rise in a laser diode junction. Forexample, for heat pulses of between 250 W and 500 W with duration ofbetween 200 μs and 1000 μs, the junction temperature can be improved byabout 3° K to 7 K in planar structures. The fin structures improve theperformance of the latent heat reservoir by achieving up to 9° Kimprovement in junction temperature when compared to a junction mounteddirectly on a Copper heat sink without the fin structures.

In some embodiments, the laser diode assembly according to any one ofthe embodiments, e.g., laser diode assembly 200 of FIG. 2, describedabove may be implemented in a v-basis package, e.g., the v-basis package100 of FIG. 1. As described in relation of FIG. 1, a v-basis packageprovides the means for mounting multiple laser diode bars in a singlepackage. Each of the laser diode bars can have multiple laser diodeassemblies included and each of the laser diode assembly can be any ofthe embodiments described above. For example, laser diode bar 22 of FIG.1 may include multiple laser diode assemblies according to any of theembodiments described above.

Although the present disclosure describes heat absorption by a metalduring phase change from solid to liquid, many other types of phasechange materials can also be used. For example, materials that absorbheat during phase change from liquid to gas can also be used to carryheat away from the laser diode. There are many types of material thatmay be suitable for use in heat abatement via liquid to gas phasetransition. For example, some of the materials that can be used in sucha liquid to gas phase transition for absorbing heat include water (in alow pressure environment), acetone, tetrahydrofuran, diethyl ether,methylene chloride, methanol, penthane, hexane, and an azeotropic mix ofwater and ethanol.

It should be noted that the examples and embodiments described hereinare for illustrative purposes only and that various modifications orchanges in light thereof will be suggested to persons skilled in the artand are to be included within the spirit and purview of this applicationand scope of the appended claims.

1. A method for thermal management in a laser diode device, the methodcomprising: providing a reservoir filled at least partially with a solidfusible metal and disposed in close proximity to a laser diode;operating the laser diode; melting the solid fusible metal; conductingheat away from the laser diode, wherein the melted fusible metal isconfined within the reservoir; ceasing operation of the laser diode; andresolidifying the fusible metal.
 2. The method of claim 1 wherein thereservoir volume is not completely occupied by the fusible metal.
 3. Themethod of claim 1 wherein the fusible metal includes Gallium.
 4. Themethod of claim 1 wherein the fusible metal is an alloy comprisingGallium, Indium, Bismuth, and Tin.
 5. The method of claim 1 whereinoperating the laser diode comprises operating the laser diode in apulsed mode.
 6. The method of claim 1 wherein the solid fusible metal ischaracterized by a thickness ranging from about 50 μm to about 250 μm.7. A method for cooling a laser diode assembly including a laser diode,the method comprising: providing a reservoir filled with a fusiblematerial and disposed in close proximity to the laser diode; operatingthe laser diode; melting a first solid portion of the fusible materialthat is closest to the laser diode to create a first molten portion;transferring the first molten portion of the solid fusible materialtowards the bottom of the reservoir; transferring a second solid portionof the fusible material from the bottom of the reservoir to position thesecond solid portion in close proximity to the laser diode; melting thesecond solid portion to create a second molten portion; andresolidifying the first molten portion to form the first solid portion.8. The method of claim 7 further comprising: transferring the secondmolten portion towards the bottom of the reservoir; transferring thefirst solid portion from the bottom of the reservoir towards the laserdiode; and resolidifying the second molten portion.
 9. The method ofclaim 7 wherein the fusible material comprises one of Gallium, paraffin,or an alloy material including Gallium, Indium, Bismuth, or Tin.
 10. Themethod of claim 7 wherein the fusible material fills only a portion ofthe reservoir.
 11. The method of claim 7 wherein operating the laserdiode comprises operating the laser diode in a pulsed mode.
 12. Themethod of claim 7 wherein resolidifying the first molten portion to formthe first solid portion comprises transferring heat from the firstmolten portion to a substrate.
 13. The method of claim 7 wherein thefusible material is characterized by a thickness ranging from about 50μm to about 250 μm.